The present invention relates to an engine, and more particularly, to a homogeneous charge compression ignition (HCCI) engine that prevents knocking and misfires and stably performs HCCI operation and a method for operating such an HCCI engine.
Conventional engines may be categorized into two types, a spark ignition (SI) engine and a diesel engine. The thermal efficiency of the ST engine may be increased by causing the air-fuel mixture to be lean. However, there is a limit to the concentration ratio at which a spark may propagate. Thus, an SI engine requires adjustment of the amount of air with a throttle valve. As a result, the thermal efficiency of the SI engine is inferior to that of a diesel engine. Conversely, a diesel engine has satisfactory thermal efficiency. However, the diesel engine does not sufficiently mix fuel and air. As a result, NOx tends to be generated due to local combustion of fuel at high temperatures, and soot tends to be generated due to local enrichment.
In comparison with such engines, an HCCI engine premixes air and fuel. Thus, the possibility of local high temperature combustion or enrichment is small, and the generated amount of NOx and soot is subtle. Further, in a homogeneous charge compression engine, chemical changes cause ignition. Thus, the dependency on the concentration ratio is lower than that of an SI engine. As a result, the HCCI engine is capable of causing the air-fuel mixture to be significantly lean, while achieving thermal efficiency at the same level as a diesel engine. With such advantages, homogeneous charge compression engines are receiving much attention. However, in a homogenous charge compression ignition engine, excessive heat would result in sudden combustion, and insufficient heat would result in misfires. Thus, in comparison to other engines, misfires, knocking, and pre-ignition are apt to occur more easily. This tends to narrow the operable range of the HCCI engine.
In an HCCI engine, a drastic increase in the concentration ratio of the mixture for output adjustment would destabilize homogeneous charge compression ignition. For example, an excessive increase in the concentration ratio would overly increase the effective pressure of the combustion chamber and result in knocking. Conversely, an excessive decrease in the concentration ratio would overly decrease the effective pressure of the combustion chamber and result in incomplete ignition of the mixture. This decreases the thermal efficiency. Thus, it is difficult to adjust the output while maintaining high efficiency when operating a conventional HCCI engine. To solve this problem, for example, Japanese Laid-Open Patent Publication No. 2002-188488 proposes a method for operating an HCCI engine that increases the output of the engine. When increasing the concentration ratio of the mixture while keeping the concentration ratio of the mixture constant; the temperature of fresh air is decreased by a predetermined amount whenever the initiation of knocking is detected in the engine to increase the engine output.
Japanese Laid-Open Patent Publication No. 2001-221075 proposes an HCCI engine including a concentration ratio setting means for setting the amount of fuel supplied from a fuel supply means and setting the concentration ratio of a premixed mixture, an intake air temperature setting means for setting the temperature of intake air supplied to the combustion chamber, and a knocking detection means. This HCCI engine also includes an output setting means for setting the output by adjusting the concentration ratio and the intake air temperature with the concentration ratio setting means and the intake air setting means based on the detection result of the knocking detection means.
The above two prior art examples detect knocking when adjusting the output and adjusts the temperature of fresh air or intake air to prevent knocking from occurring continuously. In other words, the two prior art examples cannot execute control that avoids knocking before it occurs. This affects the durability of the engine in an undesirable manner.
Further, when HCCI operation is performed under conditions enabling stable HCCI in correspondence with the required load, the load or air-fuel ratio may change due to one reason or another. Continuous HCCI operation in such a state may cause knocking or misfires. However, the two prior art examples do not take into consideration such states.